Research ReportInherited tertiary hypothyroidism in Sprague–Dawley rats
Introduction
The two principle thyroid hormones are T3 or l-3,5,3′-triiodothyronine and T4 or l -3,5,3′,5′-tetraiodothyronine. T3 is the biologically active hormone and T4, the major thyroid hormone that is secreted from the thyroid gland, is considered a precursor or prohormone (Choksi et al., 2003).The thyroid gland and thyroid hormones are central to human and animal development. The essential function of T3 is to regulate carbohydrate and protein metabolism in all cells. Changes in T3 can affect all organ systems of the body with profound effects on the cardiovascular, nervous, immune and reproductive systems. In developing mammals, the thyroid regulates growth and metabolism and plays a critical role in tissue development and differentiation (Choksi et al., 2003, Harvey and Williams, 2002).
Development of the central nervous system (CNS) in mammals proceeds through precisely determined, well-defined events in time and location. Most of these events are determined by genetic factors. Nevertheless, epigenetic factors, such as hormones and growth factors, are also important because they act to control the timing and coordination of mechanistically unrelated processes (Bernal et al., 2003). Deficiency of thyroid hormones during a critical period of development of the CNS in mammals leads to profound and potentially irreversible defects in brain maturation, and clinical syndromes arising from thyroid hormone deficiency during fetal and postnatal periods are well recognized in humans and animals (Porterfield and Hendrich, 1993, Delange, 1997).
In the rat, thyroid hormone deficiency causes a series of abnormalities in the CNS, where alteration of cell migration, neuronal differentiation and demyelination are principal findings (Bernal et al., 2003, Porterfield and Hendrich, 1993, Delange, 1997, Lucio et al., 1997, Oppenheimer and Schwartz, 1997). Thyroid hormones appear to regulate those processes associated with terminal brain differentiation such as neuronal migration, dendritic and axonal growth, synaptogenesis and myelination (Bernal, 2002). The distinguishing feature of the cerebral cortex and hippocampus in hypothyroid rats is the retarded development of the neuropil, which is characterized by smaller and more tightly packed peripheral and central neuronal cell bodies. In addition, there is a reduced synaptogenesis manifested by diminished axonal growth and dendritic outgrowth, elongation and branching, and distribution of dendritic spines. In the cerebellum, thyroid hormone deficiency results in delayed proliferation and migration of granule cells from the external germinal layer, stunted dendritic arborization and ectopic localization of the Purkinje cells, and diminished axonal myelination. Faivre et al. attributed the cause of delayed cellular migration and neuronal differentiation to a decrease in the microtubule number in Purkinje cells (Faivre et al., 1984). Developmental retardation of the rat brain can be prevented if administration of thyroid hormone is started before the end of the second week after birth (Legrand, 1986, Eayrs, 1971).
Temporal patterns of thyroid hormone-dependent gene expression in the brain suggest that the critical period of thyroid hormone sensitivity for cerebellum development is limited to the first 2–3 postnatal weeks in the rat (Bernal et al., 2003). Studies of thyroid hormone's effects on brain development have employed several mammalian species, and the most extensive and detailed investigations have focused on the neonatal rat. In general, hypothyroidism in rats was induced in pregnant mothers or neonatal rats by administering various chemicals such as propylthiouracil and methimazole in drinking water, food or through intraperitoneal (IP) injection, which will block the oxidation of iodide to iodine and consequently block the formation of T3 and T4 (Bernal et al., 2003, Porterfield and Hendrich, 1993, Delange, 1997, Lucio et al., 1997).
Our work presents an inherited autosomal recessive thyroid-deficient neurological disorder affecting a colony of Sprague–Dawley rats (Harlan Inc. IN) maintained at Texas A&M University, Lab Animal Facility. In this rat model, the thyroid hormonal (TH) assay consistently showed a significant decrease in thyrotropin releasing hormone (TRH) mRNA and protein, thyroid stimulating hormone (TSH), T3 and T4 levels in the affected rats when compared to their control littermates. These TH findings suggest a tertiary type of hypothyroidism in this animal model. On histopathological examination the CNS demonstrated progressive widespread neuronal changes, demyelinization and astrogliosis. Our present investigation of this inherited hypothyroid condition in the Sprague–Dawley (SD) rat confirmed previously reported findings of chemically-induced hypothyroidism. In addition, our study reveals new morphological and biochemical findings not previously reported, such as severe multisystem neuronal post-synaptic changes and abnormal differentiation in the adrenal medulla. To our best knowledge, there is no previous report of spontaneously inherited tertiary hypothyroidism in the rat.
Section snippets
CNS pathology in thyroid hormone deficient rats
This neurological disorder caused by hypothyroidism, affects young rats during the first month of life. The clinical manifestation was characterized by progressive development of ataxia, spasticity, weight loss and hypercholesterolemia which are noticed at 14 day's postnatal (dpn). The severity of the disease increased gradually and rats died at 30 dpn due to severe neurological deficits. On necropsy examination, there is no visible gross morphological alteration of the central nervous system
Discussion
The rat is born with an incompletely developed hypothalamus–pituitary–thyroid axis, and any changes that affect this axis could result in hypothyroidism or hyperthyroidism. The thyroid hormones, thyroxine T4 and T3 are essential to mammalian brain development and metabolic homeostasis. Congenital hypothyroidism in rats results in delayed maturation of neurons and glial cells, poor connectivity among neurons, changes in microtubules and extracellular matrix content, alteration in myelin
Reagents
Chemicals, stains and solutions were obtained from Sigma Chemical Company (St. Louis, MO, USA) unless otherwise indicated.
Animals
Ataxic and control Sprague–Dawley (SD) rats were obtained from a colony maintained since October 2005 at Texas A&M University. The founding group of 8 pairs of SD rats was obtained from Harlan Inc, IN, and one female delivered ataxic pups (4 out of 12). We bred the same dam with the same sire, then brothers and sisters and also daughters with their grandfathers. The
Acknowledgments
This work was supported from the NIH grant no. 1 R01 NS046214-01, Amd. 03, and in part from an internal grant from the Department of Veterinary Pathobiology, Texas A&M University, College Station, TX. We thank Ross Payne and Sarah Jones for their technical assistance.
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